1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device that can be miniaturized and improved in characteristics.
2. Description of the Background Art
Semiconductor memory devices such as DRAM (Dynamic Random Access Memory) are conventionally known (see Japanese Patent Laying-Open No. 9-64308, for example).
In the semiconductor memory device described in Japanese Patent Laying-Open No. 9-64308, in a memory array configuration in matrix, sense amplifier driving circuits for driving sense amplifiers and I/O control circuits for outputting data from the sense amplifiers are arranged at a plurality of intersection regions of subword drivers dispersedly arranged in a word line direction of the memory array (between memory arrays) and sense amplifiers dispersedly arranged in a data line direction of the memory array (between memory arrays) (intersection regions of strip-like regions partitioning memory arrays in the word line direction and strip-like regions partitioning memory arrays in the data line direction). If the sense amplifier driving circuits and the like are concentratedly arranged at the intersection regions in this manner, a total area for the regions provided with memory arrays, subword drivers, sense amplifiers, and the like may effectively reduced.
With further miniaturization of semiconductor devices, however, the width of the region provided with subword driver has become narrower. Moreover, with improvement of an element material forming a memory cell in a memory cell array, the number of division (the number of memory cell arrays) in the word line direction tends to be reduced. Therefore it has become difficult to arrange all the elements constituting a sense amplifier driving circuit at an intersection region.
A part of the elements constituting a sense amplifier driving circuit may be arranged in a region in which a sense amplifier is arranged (a region in which an element forming a sense amplifier (a sense amplifier-forming element) is formed). In this case, in order to minimize the total area, it is necessary to arrange components such as sense amplifier-forming elements and contact holes for fixing well potential, as densely as possible. In this way, the elements constituting sense amplifier driving circuits can well be accommodated, while the total area can be reduced to some extent. As a result of efficiently arranging the components in the region in which the sense amplifier is arranged as described above, however, the shapes of N-well and P-well that are required to arrange the element such as P-channel MOS transistor and N-channel transistor have often been complicated. As a result, the distances between a plurality of transistors (elements) constituting a sense amplifier and a well boundary portion are varied. Here, the inventor has found that if the distance between the well boundary portion and the transistor is small to some extent as described later, the threshold voltage of the transistor varies with the distance between the well boundary portion and the transistor. It follows that if the distances between a plurality of transistors (elements) and the well boundary portion are varied as described above, the characteristics (the threshold voltage) of the transistors forming the sense amplifier are varied. This may lead to degradation in characteristic of the semiconductor memory device.